Page 102 - Optofluidics Fundamentals, Devices, and Applications

Optofluidic Trapping and Transport Using Planar Photonic Devices 83

dependence on particle radius, which is three powers greater
than the best techniques currently used for microfluidic sepa-
rations. We describe how this can be exploited to develop
chromatography systems that are at least an order of magni-
tude more resolute than the state of the art.
3. Extremely high optical trapping stability: As alluded to in Sec. 5-1-4,
the trapping force is proportional to the gradient in the inten-
sity and the extremely high decay rate of the optical energy in
the near field outside the waveguide can lead to a very high
trapping force.
4. Insensitivity to surface/solution conditions: As mentioned in
Sec. 5-1-1, electrokinetic techniques are compatible only with
a limited class of fluids, exhibit extreme sensitivity to surface
conditions, and are difficult to use with semiconductor sub-
strates such as silicon (as it relies on an insulating substrate).
Optofluidic transport is much less dependent on these condi-
tions and can be used in a broader class of systems.
5. Ability to exploit techniques and components already developed
by the telecommunications industry: Over the past 20 years,
billions of dollars have been spent on research and develop-
ment in the optical communications industry yielding very
well-developed highly integrated device architectures and
cheap low-power active components. Optofluidic transport
allows us to exploit these already optimized techniques for
microfluidics.

5-3 Demonstrations of Optofluidic Transport
Prior to expanding on the advantages in the next section (Sec. 5-4) we
present a review of experimental literature on the subject in order to
better familiarize the reader with the state of the art in the technology.
Section 5-3-1 reviews the use of liquid-core and solid-core wave-
guides for optofluidic transport. In the final section we provide a
more detailed review of our recently published [49] system with suf-
ficient detail for the reader to develop their own implementations.

5-3-1 Optofluidic Transport within Solid- (and Liquid-) Core
Waveguiding Device
Recently there have been a number of researchers who have pub-
lished works on near-field optical manipulation methods (see Dhola-
kia and Reece [38] for a recent review) such as those based on the use
of surface plasmonic resonances [39,40] or other evanescent field
techniques [41]. For example, Cizmar et al. [42] demonstrated the
short-range manipulation (in the order of 40 μm) and sorting of